[wp-hackers] Adding a custom GET parameter to a feed URL

[dave morris]

I am writing a custom feed that I would like to be able to accept a
custom GET parameter. However, when I specify that extra parameter,
the feed redirects to the main blog page, and includes that custom
parameter.

For example:

http://example.com/feed/custom-feed-type/ - outputs a feed in my
custom format
http://example.com/feed/custom-feed-type/?cat=test - outputs a feed in
my custom format containing posts assigned to category 'test'
http://example.com/feed/custom-feed-type/?customparam=test - redirects
to the main blog home page

What hook can I provide to get around this redirect? I'm having
trouble finding it in the code.

Thanks,
Dave